1. Field of Invention
The present invention relates to measuring magnetic susceptibility of material. More particularly, the present invention relates to measuring magnetic susceptibility of material by a superconductive quantum interference device.
2. Description of Related Art
The superconductive quantum interference device (SQUID) has been known with the high capability for sensing magnetic flux, so that many kinds of magnetic detection systems using the SQUIDs have been developed. With its high sensitivity on magnetic flux, SQUID element has been applied to detect very small magnetic signal, such as the magnetic signals generated by bio-activity or nano-blocks, for example.
FIGS. 1A-1C are drawings, schematically illustrating several types of the SQUID design. In FIG. 1A, A SQUID 100 is usually fabricated on a substrate. The substrate has a boundary 101. The boundary 101, for example, forms two grain regions 102a and 102b with a grain boundary. Alternatively for example, the two regions 102a and 102b may have a step height to form a step boundary. The SQUID 100 has the superconductive film as shown in FIG. 1A by shading. The SQUID 100 includes two Josephson junctions 110 connected in parallel. The electrode lead 104a is deposited on the substrate at the region 102a, usually having two lead terminals. One terminal I 108 is for applying a current through the Josephson junctions 110 and the other terminal V 106 is for detecting an induced voltage signal. The electrode lead 104b is grounded.
The basic characteristics of SQUID is introduced as follows. When a bias current a little higher than the critical current is injected at the terminal I 108 and flows through the Josephson junctions 110, the Josephson junctions become resistively shunted junctions and a voltage across the Josephson junctions occurs. Due to the Meissener effect of superconductive material, when an external magnetic flux is shone onto a SQUID, a circulating current through these two junctions is induced to compensate the external magnetic flux within the area enclosed with the superconducting ring having these two Josephson junctions. Thus, an induced current by the external magnetic flux is generated to flow through the effectively shunted resistors of the Josephson junctions. As a result, the voltage detected at the terminal V 106 varies due to the application of an external magnetic flux. The voltage cross the junctions is a periodic function in response to the applied magnetic flux.
In FIG. 1B, with the SQUID design in FIG. 1A, two superconductive coils are connected to the SQUID element so as to have two detection regions 90a and 90b. If the magnetic field is not uniform, then the detection regions 90a and 90b produces unbalance effect. As a result, a magnetic gradient can be detected by the gradiometer. The proceeding SQUID is operated in directly biased current (DC) mode. However, further example in FIG. 1C, a radio-frequency (RF) SQUID magnetometer can be designed. The LC resonant circuit 94 can detect the magnetic field shone onto the superconductive coil 92 with a Josephson junction inserted in the ring. Further, if the superconductive coil 92 is replaced with two coils, which share a common Josephson junction, then a RF SQUID gradiometer can be obtained. The SQUID can be designed in various ways.
By utilizing the ability of SQUID to sense the magnetic flux, especially very weak magnetic flux, various SQUID-based systems have been developed in different aspect of applications, such as magnetocardography, magnetoencephalography, non-destructive detection, picovoltmeter, etc. However, the property of SQUID-based AC magnetic susceptibility χAC has not yet been considered. The present invention is more directed to the measurement of AC magnetic susceptibility χAC.